Adaptive linefeed control for varying load on a subscriber loop

ABSTRACT

A linefeed on a subscriber loop is generated to generate a current on the subscriber loop. A relationship between a voltage on the subscriber loop and a threshold voltage is identified. A current limit for the subscriber loop is modified in response to the identifying. The linefeed on the subscriber loop is controlled to modify the current on the subscriber loop in response to the modifying.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the field of telecommunications. More particularly, the present invention relates to the field of linefeed control for subscriber loops.

[0003] 2. Description of Related Art

[0004] Subscriber line (or loop) interface circuits (SLICs) are typically found in or near the central office exchange of a telecommunications network.

[0005] One typical SLIC provides a communications interface between a digital switching network for a central office exchange and an analog subscriber line. The analog subscriber line connects to subscriber equipment, such as a subscriber station or telephonic instrument for example, at a location remote from the central office exchange. The analog subscriber line and subscriber equipment form a subscriber loop.

[0006] The SLIC detects and transforms voiceband communications transmitted from the subscriber equipment in the form of low voltage analog signals on the subscriber loop into corresponding digital data for transmission to the digital switching network. For bi-directional communication, the SLIC also transforms digital data received from the digital switching network into corresponding low voltage analog signals for transmission on the subscriber loop to the subscriber equipment.

[0007] The SLIC also typically generates a linefeed on the subscriber loop to power the subscriber equipment. When a conventional telephone, for example, on the subscriber loop is off-hook, the SLIC should provide adequate direct current (DC) feed on the subscriber loop to help ensure proper operation of the telephone. When more than one telephone is off-hook on the same subscriber loop, however, one or more of the off-hook telephones may not receive adequate power.

[0008] The SLIC may be designed to provide adequate power to accommodate a larger load on the subscriber loop. The SLIC, however, would then unnecessarily waste power whenever a smaller load, such as only one off-hook telephone for example, is on the subscriber loop. Also, the SLIC would have to be designed to dissipate a maximum possible amount of wasted power to account for times when only a smaller load is on the subscriber loop. This maximum possible amount of wasted power typically increases as the load the SLIC is designed to support increases.

[0009] The SLIC may alternatively be designed to provide a resistive DC feed on the subscriber loop so that the power supplied to the subscriber equipment on the subscriber loop is generally proportional to the DC load on the subscriber loop. As additional telephones, for example, are switched off-hook on the same subscriber loop, the increasing DC load presented on the subscriber loop will draw from the SLIC an increasing amount of power generally in proportion to the DC load. Providing a typical resistive DC feed on a subscriber loop, however, generally elevates power dissipation for all DC load conditions and perturbs voice impedance synthesis performed by the SLIC.

SUMMARY

[0010] A method comprises generating a linefeed on a subscriber loop to generate a current on the subscriber loop, identifying a relationship between a voltage on the subscriber loop and a threshold voltage, modifying a current limit for the subscriber loop in response to the identifying, and controlling the linefeed on the subscriber loop to modify the current on the subscriber loop in response to the modifying.

[0011] The generating may comprise controlling a current source to generate a generally constant current on the subscriber loop. The identifying may comprise identifying a relationship between a metallic voltage between a first line and a second line of the subscriber loop and the threshold voltage.

[0012] The modifying may comprise increasing the current limit if the voltage on the subscriber loop is less than the threshold voltage. The modifying may comprise decreasing the current limit if the voltage on the subscriber loop is greater than the threshold voltage. The modifying may comprise increasing or decreasing the current limit by a predetermined amount.

[0013] The identifying may comprise identifying a relationship between the voltage on the subscriber loop and a range of voltages. The method may comprise modifying the range of voltages in response to the identifying. The modifying the range of voltages may comprise decreasing an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is less than the lower threshold voltage. The modifying the range of voltages may comprise increasing an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is greater than the upper threshold voltage. The modifying the range of voltages may comprise decreasing the difference between an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is less than the lower threshold voltage.

[0014] The method may be performed in response to the execution by a machine of instructions stored on a machine-readable medium.

[0015] An apparatus comprises linefeed driver circuitry to generate a linefeed on a subscriber loop to generate a current on the subscriber loop, subscriber loop sensing circuitry to sense one or more voltages on the subscriber loop, and adaptive linefeed control circuitry to monitor a voltage based on the sensed voltage(s), to identify a relationship between the monitored voltage and a threshold voltage, to modify a current limit for the subscriber loop in response to the identifying, and to control the linefeed on the subscriber loop to modify the current on the subscriber loop in response to the modifying.

[0016] The apparatus may comprise programmable registers to store values to identify the current limit and the threshold voltage. The adaptive linefeed control circuitry may be implemented with a digital signal processor.

[0017] The linefeed driver circuitry may comprise a current source to generate a generally constant current on the subscriber loop. The adaptive linefeed control circuitry may monitor a metallic voltage between a first line and a second line of the subscriber loop.

[0018] The adaptive linefeed control circuitry may modify the current limit for the subscriber loop by increasing the current limit if the monitored voltage is less than the threshold voltage. The adaptive linefeed control circuitry may modify the current limit for the subscriber loop by decreasing the current limit if the monitored voltage is greater than the threshold voltage. The adaptive linefeed control circuitry may modify the current limit for the subscriber loop by increasing or decreasing the current limit by a predetermined amount.

[0019] The adaptive linefeed control circuitry may identify a relationship between the monitored voltage and a range of voltages. The adaptive linefeed control circuitry may modify the range of voltages in response to the identifying. The adaptive linefeed control circuitry may decrease an upper threshold voltage and a lower threshold voltage for the range of voltages if the monitored voltage is less than the lower threshold voltage. The adaptive linefeed control circuitry may increase an upper threshold voltage and a lower threshold voltage for the range of voltages if the monitored voltage is greater than the upper threshold voltage. The adaptive linefeed control circuitry may decrease the difference between an upper threshold voltage and a lower threshold voltage for the range of voltages if the monitored voltage is less than the lower threshold voltage.

[0020] An apparatus comprises means for generating a linefeed on a subscriber loop to generate a current on the subscriber loop, means for identifying a relationship between a voltage on the subscriber loop and a threshold voltage, means for modifying a current limit for the subscriber loop in response to the identifying, and means for controlling the linefeed on the subscriber loop to modify the current on the subscriber loop in response to the modifying.

[0021] The modifying means may comprise means for increasing or decreasing the current limit by a predetermined amount.

[0022] The identifying means may comprise means for identifying a relationship between the voltage on the subscriber loop and a range of voltages. The apparatus may comprise means for modifying the range of voltages in response to the identifying.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] One or more embodiments of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:

[0024]FIG. 1 illustrates, for one embodiment, a subscriber line interface circuit (SLIC) coupling multiple devices on a subscriber loop to a switching network;

[0025]FIG. 2 illustrates, for one embodiment, a flow diagram to generate and control a linefeed on a subscriber loop based on a varying load on the subscriber loop;

[0026]FIG. 3 illustrates, for one embodiment, an initial subscriber loop direct current (DC) linefeed characteristic;

[0027]FIG. 4 illustrates, for one embodiment, an adaptive subscriber loop DC linefeed characteristic that accounts for a varying load on a subscriber loop;

[0028]FIG. 5 illustrates, for another embodiment, a flow diagram to generate and control a linefeed on a subscriber loop based on a varying load on the subscriber loop;

[0029]FIG. 6 illustrates, for another embodiment, an adaptive subscriber loop DC linefeed characteristic that accounts for a varying load on a subscriber loop;

[0030]FIG. 7 illustrates, for one embodiment, circuitry for a SLIC to generate and control a linefeed on a subscriber loop based on a varying load on the subscriber loop; and

[0031]FIG. 8 illustrates, for one embodiment, circuitry for a SLIC to generate and control a linefeed on a subscriber loop based on a varying load on the subscriber loop.

DETAILED DESCRIPTION

[0032] The following detailed description sets forth an embodiment or embodiments in accordance with the present invention for adaptive linefeed control for varying load on a subscriber loop.

Exemplary Subscriber Line Interface Circuit (SLIC) Environment

[0033]FIG. 1 illustrates, for one embodiment, a subscriber line interface circuit (SLIC) 100. SLIC 100 provides a communications interface between a switching network 110 and a subscriber loop 120 having multiple devices 131, 132, and 133. Switching network 110 for one embodiment is a digital switching network for a larger telecommunications network. SLIC 100 may be used for any suitable application such as, for example, terminal adaptors, cable telephony, private branch exchange (PBX) systems, key telephone systems, wireless local loop, voice over Internet protocol (IP), and integrated access devices.

[0034] Subscriber loop 120 for one embodiment, as illustrated in FIG. 1, is defined by a first line 121, a second line 122, and one or more devices 131, 132, and/or 133. For one embodiment where SLIC 100 provides an analog telephone interface, first line 121 corresponds to a tip line and second line 122 corresponds to a ring line.

[0035] Devices 131, 132, and 133 are electrically coupled in a parallel relationship between first line 121 and second line 122. Each device 131, 132, and 133 may comprise any suitable circuitry to communicate with SLIC 100 in any suitable manner. SLIC 100 and each device 131, 132, and 133 for one embodiment may transmit and/or receive any suitable analog signals. For one embodiment, each device 131, 132, and 133 is an analog telephonic device. For one embodiment, each device 131, 132, and 133 is an analog telephone. Although described in the context of three devices 131, 132, and 133, subscriber loop 120 may have any suitable number of devices, such as one, two, four, or more.

[0036] SLIC 100 may receive communications in the form of signals on subscriber loop 120 from device 131, 132, and/or 133 and forward the received signals or transform and transmit the received signals to switching network 110. SLIC 100 may receive communications in the form of signals from switching network 110 and forward the received signals or transform and transmit the received signals on subscriber loop 120 to device 131, 132, and/or 133.

[0037] For one embodiment where SLIC 100 provides an analog telephone interface to subscriber loop 120 and where switching network 110 is a digital switching network, SLIC 100 receives voiceband communications transmitted from device 131, 132, and/or 133 in the form of low voltage analog signals on subscriber loop 120 and transforms them into corresponding digital data for transmission to switching network 110. For bi-directional communication, SLIC 100 also transforms digital data received from switching network 110 into corresponding low voltage analog signals for transmission on subscriber loop 120 to device 131, 132, and/or 133.

[0038] SLIC 100 may also transmit on subscriber loop 120 to device 131, 132, and/or 133 and/or receive on subscriber loop 120 from device 131, 132, and/or 133 any suitable handshaking or control signals.

[0039] SLIC 100 for one embodiment may comprise any suitable circuitry to perform any suitable BORSCHT functions and/or any other suitable functions. BORSCHT is an acronym for battery feed, overvoltage protection, ring, supervision, coder/decoder (codec), hybrid, and test.

[0040] SLIC 100 may perform the battery feed function, for example, to provide power to device 131, 132, and/or 133 on subscriber loop 120. SLIC 100 for one embodiment receives power from a battery 104, as illustrated in FIG. 1, to provide power over subscriber loop 120. SLIC 100 for another embodiment may receive power from any other suitable source to provide power over subscriber loop 120.

[0041] SLIC 100 may perform the overvoltage protection function, for example, to protect any circuitry in or coupled to switching network 110 against voltage transients that may occur on subscriber loop 120.

[0042] SLIC 100 may perform the ring function, for example, to signal device 131, 132, and/or 133 for call progress. Where device 131, for example, is a telephone, SLIC 100 may use the ring function to ring the telephone.

[0043] SLIC 100 may perform the supervision function, for example, to detect service requests. SLIC 100 may perform the supervision function to detect when device 131, 132, and/or 133 is activated on subscriber loop 120. Where device 131, for example, is a conventional telephone, SLIC 100 may detect when the telephone is switched off-hook. SLIC 100 may also perform the supervision function, for example, to supervise calls in progress and to detect dialing input signals.

[0044] SLIC 100 may perform the codec function, for example, to encode data signals in a manner suitable for transmission by switching network 110 and to decode data signals received from switching network 110. SLIC 100 for one embodiment may use pulse code modulation, for example, to encode voiceband data, for example.

[0045] SLIC 100 may perform the hybrid function, for example, to adapt two-wire signaling, for example, for subscriber loop 120 into separate transmit and receive signaling, for example, for switching network 110.

[0046] SLIC 100 may perform the test function, for example, to test for faults and/or to indicate faults that may exist in subscriber loop 120 and/or in SLIC 100 itself.

Adaptive Linefeed Control

[0047] In performing the battery feed function, SLIC 100 for one embodiment may generate a linefeed over subscriber loop 120 to power device 131, 132, and/or 133.

[0048] Because any one, two, or all three of devices 131, 132, and 133 may be active on subscriber loop 120 at a given time, the load on subscriber loop 120 may vary as one or more devices 131, 132, and/or 133 are activated and deactivated. Generally, the load on subscriber loop 120 increases as more devices are activated on subscriber loop 120 and decreases as more devices are deactivated on subscriber loop 120.

[0049] As one example where devices 131, 132, and 133 are conventional telephones, any one telephone may be activated by switching the telephone from the on-hook state to the off-hook state to initiate a call or to receive an incoming call. The first off-hook telephone presents an initial load on subscriber loop 120. When a second telephone on subscriber loop 120 is switched to the off-hook state while the first telephone is still off-hook, both the first and second off-hook telephones present on subscriber loop 120 a load larger than the load initially presented by only the first off-hook telephone. The load on subscriber loop 120 may be further increased by switching the third telephone off-hook while the first and second telephones are still off-hook. As each telephone is deactivated by being switched to the on-hook state, the load on subscriber loop 120 decreases.

[0050] SLIC 100 for one embodiment adaptively controls the linefeed on subscriber loop 120 to account for variations in load on subscriber loop 120. SLIC 100 for one embodiment generates a linefeed on subscriber loop 120 to generate a current on subscriber loop 120 and monitors the load of subscriber loop 120. SLIC 100 for one embodiment then controls the linefeed on subscriber loop 120 to modify the current on subscriber loop 120 based on the monitored load. SLIC 100 for one embodiment may increase the current provided by the linefeed as the monitored load increases and may decrease the current provided by the linefeed as the monitored load decreases. In this manner, SLIC 100 may provide adequate power not only when only one device 131, 132, or 133 is activated on subscriber loop 120 but also when multiple devices 131, 132, and/or 133 are activated on subscriber loop 120 with minimized or reduced concern for having to dissipate excess power.

[0051] SLIC 100 may adaptively control the linefeed on subscriber loop 120 to account for variations in load on subscriber loop 120 in any suitable manner. For one embodiment, SLIC 100 may adaptively control the linefeed on subscriber loop 120 in accordance with a flow diagram 200 of FIG. 2.

[0052] For block 202 of flow diagram 200, SLIC 100 sets a range of subscriber loop metallic voltages V_(M). The metallic voltage V_(M) for subscriber loop 120 is the voltage difference between first line 121 and second line 122 of subscriber loop 120. SLIC 100 for one embodiment may attempt to generate and maintain a subscriber loop metallic voltage V_(M) in the set range despite variations in the load on subscriber loop 120. Any suitable range of subscriber loop metallic voltages V_(M) may be used.

[0053] SLIC 100 for one embodiment may set the range of subscriber loop metallic voltages V_(M) by storing in one or more registers of SLIC 100 one or more values that may be used to identify an upper metallic voltage threshold V_(UTH) and a lower metallic voltage threshold V_(LTH). SLIC 100 for one embodiment may store the upper metallic voltage threshold V_(UTH) and the lower metallic voltage threshold V_(LTH) in one or more registers of SLIC 100. SLIC 100 for another embodiment may store in one or more registers of SLIC 100 a target subscriber loop metallic voltage V_(T), an upper voltage offset amount or percentage from the target subscriber loop metallic voltage V_(T) to identify an upper metallic voltage threshold V_(UTH), and a lower voltage offset amount or percentage from the target subscriber loop metallic voltage V_(T) to identify a lower metallic voltage threshold V_(LTH). The upper voltage offset amount or percentage and the lower voltage offset amount or percentage for one embodiment may be the same, allowing both the upper and lower voltage offset amounts or percentages to be stored with only a single value. The one or more values that may be used to identify an upper metallic voltage threshold V_(UTH) and a lower metallic voltage threshold V_(LTH) may be predetermined and/or programmable.

[0054] For block 204, SLIC 100 sets a subscriber loop current limit I_(LIM) to an initial value I_(LIM1). SLIC 100 for one embodiment may attempt to generate and maintain on subscriber loop 120 a current I_(LOOP) approximately equal to a set current limit I_(LIM) for a given load or range of loads on subscriber loop 120. The initial subscriber loop current limit I_(LIM1) may have any suitable value. The initial subscriber loop current limit I_(LIM1) for one embodiment has a value that would help provide adequate power for one or more activated devices 131, 132, and/or 133 on subscriber loop 120 with reduced or minimized concern for having to dissipate excess power.

[0055] SLIC 100 for one embodiment may set the initial subscriber loop current limit I_(LIM1) by storing in one or more registers of SLIC 100 one or more values that may be used to identify the initial subscriber loop current limit I_(LIM1). SLIC 100 for one embodiment may store the initial subscriber loop current limit I_(LIM1) in a register of SLIC 100. The one or more values that may be used to identify an initial subscriber loop current limit I_(LIM1) may be predetermined and/or programmable.

[0056] For block 206, SLIC 100 generates a subscriber loop open circuit metallic voltage V_(OC). SLIC 100 for one embodiment may generate the subscriber loop open circuit metallic voltage V_(OC) such that the metallic voltage V_(M) for subscriber loop 120 is approximately equal to the subscriber loop open circuit metallic voltage V_(OC) when subscriber loop 120 is in an open circuit state. Subscriber loop 120 for one embodiment is in an open circuit state when all devices 131, 132, and 133 on subscriber loop 120 are deactivated.

[0057] SLIC 100 may generate any suitable subscriber loop open circuit metallic voltage V_(OC) in any suitable manner. SLIC 100 for one embodiment may generate a subscriber loop open circuit metallic voltage V_(OC) in the range of approximately 0 volts (V) to approximately 90-100 volts (V). SLIC 100 for one embodiment may generate a subscriber loop open circuit metallic voltage V_(OC) of approximately 48 volts (V), for example.

[0058] SLIC 100 for one embodiment may set the subscriber loop open circuit metallic voltage V_(OC) by storing in one or more registers of SLIC 100 one or more values that may be used to identify the subscriber loop open circuit metallic voltage V_(OC). SLIC 100 for one embodiment may store the subscriber loop open circuit metallic voltage V_(OC) in a register of SLIC 100. The one or more values that may be used to identify a subscriber loop open circuit metallic voltage V_(OC) may be predetermined and/or programmable.

[0059] When at least one device 131, 132, and/or 133 is activated on subscriber loop 120 for block 208, SLIC 100 for block 210 generates a linefeed on subscriber loop 120 to help power any activated device 131, 132, and/or 133. SLIC 100 for one embodiment may generate a linefeed on subscriber loop 120 using a current source to generate a current on subscriber loop 120. SLIC 100 for another embodiment may generate a linefeed on subscriber loop 120 by generating a suitable metallic voltage V_(M) for subscriber loop 120. SLIC 100 for one embodiment may generate a direct current (DC) linefeed on subscriber loop 120.

[0060] SLIC 100 for one embodiment may maintain the subscriber loop open circuit metallic voltage V_(OC) on first and second lines 121 and 122 for block 206. When activation of a device 131, 132, or 133 on subscriber loop 120 for block 208 closes the open circuit of subscriber loop 120, SLIC 100 consequently generates a linefeed on subscriber loop 120 for block 210.

[0061]FIG. 3 illustrates, for one embodiment, a characteristic of an initial subscriber loop direct current (DC) linefeed generated by SLIC 100. As illustrated in FIG. 3, SLIC 100 for one embodiment initially generates an open circuit metallic voltage V_(OC) on subscriber loop 120. When SLIC 100 generates the linefeed on subscriber loop 120, the subscriber loop metallic voltage V_(M) initially decreases generally linearly along a curve 302 as the subscriber loop current I_(LOOP) increases due to an inherent DC output resistance R_(O) of SLIC 100. The slope of curve 302 is approximately equal to the additive inverse of the DC output resistance R_(O). SLIC 100 may have any suitable DC output resistance R_(O).

[0062] For block 212, SLIC 100 controls the linefeed on subscriber loop 120 such that the subscriber loop current I_(LOOP) is approximately equal to the set subscriber loop current limit I_(LIM). SLIC 100 for one embodiment controls the linefeed on subscriber loop 120 to attempt to maintain a generally constant current I_(LIM) to help provide adequate power for one or more activated devices 131, 132, and/or 133 on subscriber loop 120 with reduced or minimized concern for having to dissipate excess power.

[0063] SLIC 100 may control the linefeed on subscriber loop 120 for block 212 in any suitable manner. SLIC 100 for one embodiment controls a current source generating current on subscriber loop 120 to help maintain the subscriber loop current I_(LOOP) at approximately the set subscriber loop current limit I_(LIM). SLIC 100 for another embodiment may monitor the subscriber loop current I_(LOOP) and/or the subscriber loop metallic voltage V_(M) and may modify the subscriber loop metallic voltage V_(M) based on the monitored subscriber loop current I_(LOOP) and/or subscriber loop metallic voltage V_(M) to help maintain the subscriber loop current I_(LOOP) at approximately the set subscriber loop current limit I_(LIM).

[0064] As illustrated in FIG. 3, SLIC 100 for one embodiment may control the linefeed on subscriber loop 120 for block 212 by controlling a current source to generate and maintain the subscriber loop current I_(LOOP) on subscriber loop 120 at approximately the set subscriber loop current limit I_(LIM). Because the subscriber loop current I_(LOOP) and the subscriber loop metallic voltage V_(M) have a generally linear relationship that depends on the total DC resistance R_(LOOP) of subscriber loop 120, including any resistance presented on subscriber loop 120 by devices 131, 132, and 133, the subscriber loop metallic voltage V_(M) will decrease along a curve 304 until the subscriber loop metallic voltage V_(M) is approximately equal to the set subscriber loop current limit I_(LIM1) multiplied by the total subscriber loop resistance R_(LOOP), or until V_(M)≈I_(LIM1)*R_(LOOP). Where the total subscriber loop resistance has a value R_(LOOP1), for example, as illustrated in FIG. 3, the subscriber loop metallic voltage V_(M) decreases along curve 304 until the subscriber loop metallic voltage V_(M) is approximately equal to V_(M1)=I_(LIM1)*R_(LOOP1).

[0065] SLIC 100 for another embodiment may initially control the linefeed on subscriber loop 120 for block 212 when the subscriber loop current I_(LOOP) nears, reaches, or attempts to exceed the initially set subscriber loop current limit I_(LIM1) SLIC 100 may then modify the subscriber loop metallic voltage V_(M) to help maintain the subscriber loop current I_(LOOP) at approximately the set subscriber loop current limit I_(LIM1). As illustrated in FIG. 3, SLIC 100 may decrease the subscriber loop metallic voltage V_(M) along curve 304 until the subscriber loop metallic voltage V_(M) is approximately equal to the set subscriber loop current limit I_(LIM1) multiplied by the total subscriber loop resistance R_(LOOP), or until V_(M)≈I_(LIM1)*R_(LOOP).

[0066] For block 214, SLIC 100 monitors the subscriber loop metallic voltage V_(M). SLIC 100 for one embodiment may monitor the subscriber loop metallic voltage V_(M) to help monitor the load on subscriber loop 120. The load on subscriber loop 120 may increase or decrease, for example, because of the activation or deactivation of any device 131, 132, and/or 133 on subscriber loop 120.

[0067] As one example where only one device 131, for example, is activated on subscriber loop 120, SLIC 100 initially controls the linefeed on subscriber loop 120 for block 212 such that the subscriber loop metallic voltage V_(M) along curve 304 is approximately equal to V_(M1)=I_(LIM1)*R_(LOOP1), where R_(LOOP1) is the total subscriber loop resistance with device 131 as the only activated device on subscriber loop 120. The relationship between the subscriber loop metallic voltage V_(M) and the subscriber loop current I_(LOOP) with device 131 as the only activated device on subscriber loop 120 is represented in FIG. 3 by a generally linear curve 306 along which the subscriber loop metallic voltage V_(M) increases as the subscriber loop current I_(LOOP) increases. The slope of curve 306 is approximately equal to the total subscriber loop resistance R_(LOOP1).

[0068] When another device 132, for example, is activated on subscriber loop 120 and increases the load on subscriber loop 120, the total subscriber loop resistance R_(LOOP) decreases. The relationship between the subscriber loop metallic voltage V_(M) and the subscriber loop current I_(LOOP) with both devices 131 and 132 as the only activated devices on subscriber loop 120 is represented in FIG. 3 by a generally linear curve 308. The subscriber loop metallic voltage V_(M) increases along curve 308 as the subscriber loop current I_(LOOP) increases yet at a slower rate as compared to curve 306. The slope of curve 308 is approximately equal to the total subscriber loop resistance R_(LOOP2), where R_(LOOP2) is the total subscriber loop resistance with both devices 131 and 132 as the only activated devices on subscriber loop 120.

[0069] Although described as having a varying load due to the activation and deactivation of devices 131, 132, and/or 133, subscriber loop 120 may also have a varying load due to, for example, any variation in the load presented by any device 131, 132, and/or 133 on subscriber loop 120 while the device 131, 132, and/or 133 is activated.

[0070] For a given subscriber loop current I_(LOOP), such as the set current limit I_(LIM) for example, the subscriber loop metallic voltage V_(M) decreases as the load on subscriber loop 120 increases and increases as the load on subscriber loop 120 decreases. SLIC 100 for one embodiment may therefore monitor the load on subscriber loop 120 by monitoring the subscriber loop metallic voltage V_(M) at a given subscriber loop current I_(LOOP). SLIC 100 for one embodiment may then increase the set subscriber loop current limit I_(LIM) as the monitored subscriber loop metallic voltage V_(M) decreases to increase the amount of power provided by the linefeed. SLIC 100 for one embodiment may decrease the set subscriber loop current limit I_(LIM) as the monitored subscriber loop metallic voltage V_(M) increases to decrease the amount of power provided by the linefeed.

[0071] SLIC 100 for one embodiment may monitor the subscriber loop metallic voltage V_(M) for block 214 to identify a relationship between the subscriber loop metallic voltage V_(M) and a threshold voltage. SLIC 100 for one embodiment may monitor the subscriber loop metallic voltage V_(M) to identify a relationship between the subscriber loop metallic voltage V_(M) and the set range of subscriber loop metallic voltages V_(M). SLIC 100 may, for example, identify whether the subscriber loop metallic voltage V_(M) is greater than the upper metallic voltage threshold V_(UTH) and less than the lower metallic voltage threshold V_(LTH). In this manner, hysteresis is introduced into the linefeed.

[0072] For block 216, SLIC 100 for one embodiment may identify whether the monitored subscriber loop metallic voltage V_(M) is less than the lower metallic voltage threshold V_(LTH) for the set range of subscriber loop metallic voltages V_(M). Although described as identifying whether the monitored subscriber loop metallic voltage V_(M) is less than the lower metallic voltage threshold V_(LTH), SLIC 100 may identify whether the monitored subscriber loop metallic voltage V_(M) and the lower metallic voltage threshold V_(LTH) satisfy any suitable condition. SLIC 100 for another embodiment may identify whether the monitored subscriber loop metallic voltage V_(M) is less than or equal to, is approaching, or is about to exceed the lower metallic voltage threshold V_(LTH).

[0073] If the monitored subscriber loop metallic voltage V_(M) is, for example, less than the lower metallic voltage threshold V_(LTH) for the set range of subscriber loop metallic voltages V_(M), SLIC 100 for block 218 increases the set subscriber loop current limit I_(LIM) SLIC 100 may increase the set subscriber loop current limit I_(LIM) by any suitable amount. SLIC 100 for one embodiment may increase the set subscriber loop current limit I_(LIM) by a predetermined amount that may be fixed for block 218 or that may be dependent, for example, on the current set subscriber loop current limit I_(LIM) SLIC 100 for another embodiment may increase the set subscriber loop current limit I_(LIM) by an amount depending on the extent which the monitored subscriber loop metallic voltage V_(M) is less than the lower metallic voltage threshold V_(LTH).

[0074] SLIC 100 may then control the linefeed on subscriber loop 120 for block 212 such that the subscriber loop current I_(LOOP) is approximately equal to the increased subscriber loop current limit I_(LIM) and continue monitoring the subscriber loop metallic voltage V_(M) for block 214.

[0075] If the monitored subscriber loop metallic voltage V_(M) is not, for example, less than the lower metallic voltage threshold V_(LTH) for the set range of subscriber loop metallic voltages V_(M), SLIC 100 for block 220 may identify whether the monitored subscriber loop metallic voltage V_(M) is greater than the upper metallic voltage threshold V_(UTH) for the set range of subscriber loop metallic voltages V_(M). Although described as identifying whether the monitored subscriber loop metallic voltage V_(M) is greater than the upper metallic voltage threshold V_(UTH), SLIC 100 may identify whether the monitored subscriber loop metallic voltage V_(M) and the upper metallic voltage threshold V_(UTH) satisfy any suitable condition. SLIC 100 for another embodiment may identify whether the monitored subscriber loop metallic voltage V_(M) is greater than or equal to, is approaching, or is about to exceed the upper metallic voltage threshold V_(UTH).

[0076] If the monitored subscriber loop metallic voltage V_(M) is, for example, greater than the upper metallic voltage threshold V_(UTH) for the set range of subscriber loop metallic voltages V_(M), SLIC 100 for block 222 decreases the set subscriber loop current limit I_(LIM) SLIC 100 may decrease the set subscriber loop current limit I_(LIM) by any suitable amount. SLIC 100 for one embodiment may decrease the set subscriber loop current limit I_(LIM) by a predetermined amount that may be fixed for block 222 or that may be dependent, for example, on the current set subscriber loop current limit I_(LIM). SLIC 100 for another embodiment may decrease the set subscriber loop current limit I_(LIM) by an amount depending on the extent which the monitored subscriber loop metallic voltage V_(M) is greater than the upper metallic voltage threshold V_(UTH).

[0077] SLIC 100 may then control the linefeed on subscriber loop 120 for block 212 such that the subscriber loop current I_(LOOP) is approximately equal to the decreased subscriber loop current limit I_(LIM) and continue monitoring the subscriber loop metallic voltage V_(M) for block 214.

[0078] SLIC 100 continues controlling the linefeed on subscriber loop 120 for block 212, monitoring the subscriber loop metallic voltage V_(M) for block 214, and modifying the subscriber loop current limit I_(LIM) for blocks 216-222 to help generate and maintain the subscriber loop metallic voltage V_(M) in the set range of subscriber loop metallic voltages V_(M) despite variations in the load on subscriber loop 120. If the monitored subscriber loop metallic voltage V_(M) is not in the set range after an increase or decrease in the set subscriber loop current limit I_(LIM), SLIC 100 for one embodiment may continue to increase or decrease, respectively, the set subscriber loop current limit I_(LIM) in repeating operations for blocks 212-222 to help modify the monitored subscriber loop metallic voltage V_(M) to be in the set range of subscriber loop metallic voltages V_(M).

[0079]FIG. 4 illustrates, for one embodiment, a characteristic of a subscriber loop direct current (DC) linefeed generated by SLIC 100 in accordance with flow diagram 200 of FIG. 2. The linefeed characteristic illustrated in FIG. 4 extends the linefeed characteristic illustrated in FIG. 3 to account for a varying load on subscriber loop 120.

[0080] As illustrated in FIG. 4, when only one device 131 is activated on subscriber loop 120, SLIC 100 initially controls the linefeed on subscriber loop 120 for block 212 such that the subscriber loop metallic voltage V_(M) along curve 304 is approximately equal to V_(M1)=I_(LIM1)*R_(LOOP1). Because the subscriber loop metallic voltage V_(M1) is less than the upper metallic voltage threshold V_(UTH) and greater than the lower metallic voltage threshold V_(LTH), the subscriber loop current I_(LIM1) is considered adequate to power device 131 with reduced or minimized concern for having to dissipate excess power.

[0081] When device 132 is activated and increases the load on subscriber loop 120, SLIC 100 controls the linefeed on subscriber loop 120 for block 212 such that the subscriber loop metallic voltage V_(M) decreases along curve 304 toward V_(M2)=I_(LIM1)*R_(LOOP2). Because the subscriber loop metallic voltage V_(M2) is less than the lower metallic voltage threshold V_(LTH), SLIC 100 increases the set subscriber loop current limit I_(LIM) from the initial current limit I_(LIM1) to a current limit I_(LIM2) and controls the linefeed for block 212 to increase the subscriber loop current I_(LOOP) to approximately the current limit I_(LIM2). Because the subscriber loop metallic voltage V_(M3)=I_(LIM2)*R_(LOOP2) is still less than the lower metallic voltage threshold V_(LTH), SLIC 100 increases the set subscriber loop current limit I_(LIM) from current limit I_(LIM2) to a current limit I_(LIM3) and controls the linefeed for block 212 to increase the subscriber loop current I_(LOOP) to approximately the current limit I_(LIM3). Because the subscriber loop metallic voltage V_(M4)=I_(LIM3)*R_(LOOP2) is less than the upper metallic voltage threshold V_(UTH) and greater than the lower metallic voltage threshold V_(LTH), the subscriber loop current I_(LIM3) is considered adequate to power both devices 131 and 132 with reduced or minimized concern for having to dissipate excess power.

[0082] When device 132 is deactivated and decreases the load on subscriber loop 120 while device 131 remains activated, SLIC 100 controls the linefeed on subscriber loop 120 for block 212 such that the subscriber loop metallic voltage V_(M) increases toward V_(M5)=I_(LIM3)*R_(LOOP1). Because the subscriber loop metallic voltage V_(M5) is greater than the upper metallic voltage threshold V_(UTH), SLIC 100 decreases the set subscriber loop current limit I_(LIM) from current limit I_(LIM3) to current limit I_(LIM2) and controls the linefeed for block 212 to decrease the subscriber loop current I_(LOOP) to approximately the current limit I_(LIM2). Because the subscriber loop metallic voltage V_(M6)=I_(LIM2)*R_(LOOP1) is less than the upper metallic voltage threshold V_(UTH) and greater than the lower metallic voltage threshold V_(LTH), the subscriber loop current I_(LIM2) is considered adequate to power device 131 with reduced or minimized concern for having to dissipate excess power that would otherwise be generated if SLIC 100 maintained the subscriber loop current I_(LOOP) at the higher subscriber loop current limit I_(LIM3) while only device 131 is activated on subscriber loop 120.

[0083] If SLIC 100 for block 220 identifies that the monitored subscriber loop metallic voltage V_(M) is, for example, greater than the upper metallic voltage threshold V_(UTH) and for block 222 decreases the set subscriber loop current limit I_(LIM), SLIC 100 for one embodiment may then identify whether any device 131, 132, and/or 133 is still activated on subscriber loop 120 for block 224. If so, SLIC 100 may then control the linefeed on subscriber loop 120 for block 212 such that the subscriber loop current I_(LOOP) is approximately equal to the decreased subscriber loop current limit I_(LIM) and continue monitoring the subscriber loop metallic voltage V_(M) for block 214.

[0084] SLIC 100 for one embodiment may continue repeating operations for blocks 212-224 until none of devices 131, 132, and 133 are activated on subscriber loop 120 for block 224. SLIC 100 for one embodiment may then reset the subscriber loop current limit I_(LIM) to its initial value I_(LIM1) for block 204, generate a subscriber loop open circuit metallic voltage V_(OC) for block 206, and wait for any device 131, 132, and/or 133 to be activated on subscriber loop 120 for block 208.

[0085] SLIC 100 for one embodiment may detect when none of devices 131, 132, and 133 are activated on subscriber loop 120 for block 224. SLIC 100 may, for example, monitor the subscriber loop current I_(LOOP) and/or the subscriber loop metallic voltage V_(M) to detect when none of devices 131, 132, and 133 are activated on subscriber loop 120.

[0086] SLIC 100 may perform operations for blocks 202-224 in any suitable order and may or may not overlap in time the performance of any suitable operation with any other suitable operation. SLIC 100 for another embodiment may, for example, perform operations for blocks 220, 222, and 224 prior to performing operations for blocks 216 and 218. As another example, SLIC 100 for another embodiment may perform operations for blocks 202, 204, and/or 206 in any suitable order. SLIC 100 for another embodiment may monitor subscriber loop 120 for blocks 208, 214, 216, 220, and/or 224 substantially continuously or may monitor subscriber loop 120 for blocks 208, 214, 216, 220 and/or 224 discretely at a suitable sampling rate. SLIC 100 for another embodiment may monitor subscriber loop 120 for blocks 214, 216, 220, and/or 224 while SLIC 100 performs operations for block 212.

[0087] SLIC 100 for another embodiment may adaptively control the linefeed on subscriber loop 120 by modifying the set range of subscriber loop metallic voltages V_(M) as SLIC 100 modifies the subscriber loop current I_(LOOP) based on the monitored load on subscriber loop 120. For one embodiment, SLIC 100 may adaptively control the linefeed on subscriber loop 120 in this manner in accordance with a flow diagram 500 of FIG. 5.

[0088] Blocks 502, 504, 506, 508, 510, 512, 514, 516, 518, 520, 522, and 524 of flow diagram 500 generally correspond to blocks 202, 204, 206, 208, 210, 212, 214, 216, 218, 220, 222, and 224 of flow diagram 200, respectively.

[0089] For flow diagram 500, however, SLIC 100 for block 502 sets an initial range of subscriber loop metallic voltages V_(M) that may be modified as SLIC 100 modifies the set subscriber loop current limit I_(LIM) in response to a varying load on subscriber loop 120.

[0090] SLIC 100 for block 518 may optionally modify the set range of subscriber loop metallic voltages V_(M) when SLIC 100 increases the set subscriber loop current limit I_(LIM) in response to identifying for block 516 that the monitored subscriber loop metallic voltage V_(M) is, for example, less than the lower metallic voltage threshold V_(LTH) for the current set range of subscriber loop metallic voltages V_(M).

[0091] SLIC 100 for block 522 may optionally modify the set range of subscriber loop metallic voltages V_(M) when SLIC 100 decreases the set subscriber loop current limit I_(LIM) in response to identifying for block 520 that the monitored subscriber loop metallic voltage V_(M) is, for example, greater than the upper metallic voltage threshold V_(UTH) for the current set range of subscriber loop metallic voltages V_(M).

[0092] When SLIC 100 identifies none of devices 131, 132, and 133 are activated on subscriber loop 120 for block 524, SLIC 100 for one embodiment may then reset the range of subscriber loop metallic voltages V_(M) to its initial range for block 502.

[0093] SLIC 100 may modify the set range of subscriber loop metallic voltages V_(M) to any range of suitable values in any suitable manner. SLIC 100 for one embodiment may modify the set range of subscriber loop metallic voltages V_(M) to a range of values as a suitable function of the set subscriber loop current limit I_(LIM).

[0094] SLIC 100 for one embodiment may modify the set range of subscriber loop metallic voltages V_(M) to provide a resistive linefeed by decreasing both the upper metallic voltage threshold V_(UTH) and the lower metallic voltage threshold V_(LTH) as SLIC 100 increases the subscriber loop current I_(LOOP) and by increasing both the upper metallic voltage threshold V_(UTH) and the lower metallic voltage threshold V_(LTH) as SLIC 100 decreases the subscriber loop current I_(LOOP).

[0095] SLIC 100 for one embodiment may modify the set range of subscriber loop metallic voltages V_(M) to provide a linefeed with a hysteresis that is dependent on the subscriber loop current I_(LOOP). SLIC 100 for one embodiment may, for example, decrease the difference between the upper metallic voltage threshold V_(UTH) and the lower metallic voltage threshold V_(LTH) as SLIC 100 increases the subscriber loop current I_(LOOP) and increase the difference between the upper metallic voltage threshold V_(UTH) and the lower metallic voltage threshold V_(LTH) as SLIC 100 decreases the subscriber loop current I_(LOOP).

[0096]FIG. 6 illustrates, for one embodiment, a characteristic of a resistive subscriber loop direct current (DC) linefeed generated by SLIC 100 in accordance with flow diagram 500 of FIG. 5.

[0097] As illustrated in FIG. 6, when only one device 131 is activated on subscriber loop 120, SLIC 100 initially controls the linefeed on subscriber loop 120 for block 512 such that the subscriber loop metallic voltage V_(M) along curve 304 is approximately equal to V_(M1)=I_(LIM1)*R_(LOOP1). Because the subscriber loop metallic voltage V_(M1) is greater than the lower metallic voltage threshold V_(LTH1) of the initial set range of subscriber loop metallic voltages V_(M), the subscriber loop current I_(LIM1) is considered adequate to power device 131 with reduced or minimized concern for having to dissipate excess power. The initial set range of subscriber loop metallic voltages V_(M) may have any suitable upper metallic voltage threshold V_(UTH1), such as the subscriber loop open circuit metallic voltage V_(OC) for example.

[0098] When device 132 is activated and increases the load on subscriber loop 120, SLIC 100 controls the linefeed on subscriber loop 120 for block 512 such that the subscriber loop metallic voltage V_(M) decreases along curve 304 toward V_(M2)=I_(LIM1)*R_(LOOP2). Because the subscriber loop metallic voltage V_(M2) is less than the lower metallic voltage threshold V_(LTH1), SLIC 100 increases the set subscriber loop current limit I_(LIM) from the initial current limit I_(LIM1) to a current limit I_(LIM2), modifies the set range of subscriber loop metallic voltages V_(M) by decreasing the upper metallic voltage threshold V_(UTH1) to V_(UTH2) and decreasing the lower metallic voltage threshold V_(LTH1) to V_(LTH2), and controls the linefeed for block 512 to increase the subscriber loop current I_(LOOP) to approximately the current limit I_(LIM2). Because the subscriber loop metallic voltage V_(M)=I_(LIM2)*R_(LOOP2) is less than the upper metallic voltage threshold V_(UTH2) and greater than the lower metallic voltage threshold V_(LTH2), the subscriber loop current I_(LIM2) is considered adequate to power both devices 131 and 132 with reduced or minimized concern for having to dissipate excess power.

[0099] When device 132 is deactivated and decreases the load on subscriber loop 120 while device 131 remains activated, SLIC 100 controls the linefeed on subscriber loop 120 for block 512 such that the subscriber loop metallic voltage V_(M) increases toward V_(M4)=I_(LIM2)*R_(LOOP1). Because the subscriber loop metallic voltage V_(M4) is less than the upper metallic voltage threshold V_(UTH2) and greater than the lower metallic voltage threshold V_(LTH2), the subscriber loop current I_(LIM2) is considered adequate to power device 131.

[0100] SLIC 100 for one embodiment may be designed for digital control of a resistive linefeed in accordance with flow diagram 500 of FIG. 5 and therefore may be manufactured in a repeatable manner and perform voice impedance synthesis with relatively fewer perturbations.

[0101] Although described as identifying a relationship between the subscriber loop metallic voltage V_(M) and a range of subscriber loop metallic voltages V_(M) and modifying the subscriber loop current limit I_(LIM) and/or the range of subscriber loop metallic voltages V_(M) to help generate and maintain the subscriber loop metallic voltage V_(M) in a desired range of subscriber loop metallic voltages V_(M), SLIC 100 for one embodiment may similarly identify a relationship between the subscriber loop metallic voltage V_(M) and a single target or threshold metallic voltage V_(T) and may modify the subscriber loop current limit I_(LIM) and/or the target or threshold metallic voltage V_(T) to help generate and maintain the subscriber loop metallic voltage V_(M) at or around a desired target or threshold metallic voltage V_(T) despite variations in the load on subscriber loop 120.

[0102] Circuitry for Adaptive Linefeed Control

[0103] SLIC 100 may comprise any suitable circuitry to generate and control the linefeed on subscriber loop 120 based on a varying load on subscriber loop 120.

[0104] As illustrated in FIG. 7, SLIC 100 for one embodiment may comprise linefeed driver circuitry 710, subscriber loop sensing circuitry 720, adaptive linefeed control circuitry 730, and one or more programmable registers 740.

[0105] Linefeed driver circuitry 710 is coupled to first line 121 and to second line 122 of subscriber loop 120 to generate and maintain a linefeed on subscriber loop 120. Linefeed driver circuitry 710 for one embodiment comprises a current source to generate and maintain a linefeed on subscriber loop 120. Linefeed driver circuitry 710 for another embodiment may generate and maintain a suitable metallic voltage V_(M) for subscriber loop 120 to generate and maintain a linefeed on subscriber loop 120. Linefeed driver circuitry 710 is coupled to battery 104 to receive power and has one or more inputs coupled to one or more outputs of adaptive linefeed control circuitry 730 to receive one or more linefeed control signals from adaptive linefeed control circuitry 730 to control the linefeed on subscriber loop 120. Linefeed driver circuitry 710 may comprise any suitable analog and/or digital circuitry.

[0106] Subscriber loop sensing circuitry 720 is coupled to first line 121 and to second line 122 of subscriber loop 120 to sense the voltage on first line 121, the voltage on second line 122, the subscriber loop metallic voltage V_(M) between first line 121 and second line 122, and/or the subscriber loop current I_(LOOP) on subscriber loop 120. Subscriber loop sensing circuitry 720 has one or more outputs coupled to one or more inputs of adaptive linefeed control circuitry 730 to output the sensed first line voltage, the sensed second line voltage, the sensed subscriber loop metallic voltage V_(M), and/or the sensed subscriber loop current I_(LOOP) to adaptive linefeed control circuitry 730. Subscriber loop sensing circuitry 720 may comprise any suitable analog and/or digital circuitry.

[0107] Subscriber loop sensing circuitry 720 may sense the first line voltage, the second line voltage, the subscriber loop metallic voltage V_(M), and/or the subscriber loop current I_(LOOP) in any suitable manner at any suitable rate. Subscriber loop sensing circuitry 720 for one embodiment may sense the subscriber loop metallic voltage V_(M) directly or by identifying the difference between the sensed first line voltage and the sensed second line voltage. Subscriber loop sensing circuitry 720 may sense the first line voltage, the second line voltage, the subscriber loop metallic voltage V_(M), and/or the subscriber loop current I_(LOOP) substantially continuously, at predetermined intervals, or sporadically, for example.

[0108] Adaptive linefeed control circuitry 730 has one or more inputs coupled to one or more outputs of register(s) 740 to receive one or more values that may be used to identify, for example, one or more upper metallic voltage thresholds V_(UTH), one or more lower metallic voltage thresholds V_(LTH), an initial subscriber loop current limit I_(LIM1), and/or a subscriber loop open circuit voltage V_(OC). Adaptive linefeed control circuitry 730 also has one or more inputs coupled to one or more outputs of subscriber loop sensing circuitry 720 to receive a sensed first line voltage, a sensed second line voltage, a sensed subscriber loop metallic voltage V_(M), and/or a sensed subscriber loop current I_(LOOP) Adaptive linefeed control circuitry 730 has one or more outputs coupled to one or more inputs of linefeed driver circuitry 710 to transmit one or more linefeed control signals to control the linefeed on subscriber loop 120. Adaptive linefeed control circuitry 730 may comprise any suitable analog and/or digital circuitry.

[0109] Adaptive linefeed control circuitry 730 for one embodiment transmits one or more linefeed control signals to control the linefeed on subscriber loop 120 based on the identified upper metallic voltage threshold(s) V_(UTH), lower metallic voltage threshold(s) V_(LTH), initial subscriber loop current limit I_(LIM1), subscriber loop open circuit metallic voltage V_(OC), sensed first line voltage, sensed second line voltage, sensed subscriber loop metallic voltage V_(M), and/or sensed subscriber loop current I_(LOOP) in accordance with flow diagram 200 of FIG. 2 or with flow diagram 500 of FIG. 5. Adaptive linefeed control circuitry 730 for one embodiment may receive a sensed subscriber loop metallic voltage V_(M) from subscriber loop sensing circuitry 720 to monitor the subscriber loop metallic voltage V_(M) for block 214 of flow diagram 200 or block 514 of flow diagram 500 or may receive a sensed first line voltage and a sensed second line voltage from subscriber loop sensing circuitry 720 and identify the difference between the sensed first line voltage and the sensed second line voltage to monitor the subscriber loop metallic voltage V_(M) for block 214 or block 514.

[0110] Adaptive linefeed control circuitry 730 and programmable register(s) 740 for one embodiment may be implemented using a digital signal processor (DSP) 830 as illustrated in FIG. 8. DSP 830 for one embodiment has a DSP interface 832 to allow DSP 830 to be controlled or programmed to adaptively control the linefeed on subscriber loop 120 in performing the battery feed function to account for variations in load on subscriber loop 120. DSP 830 for one embodiment may also be controlled or programmed to help perform one or more other functions, such as one or more other BORSCHT functions in addition to the battery feed function. DSP interface 832 may also be used to program one or more values in one or more registers in DSP 830 to set one or more control parameters for one or more functions performed by DSP 830.

[0111] SLIC 100 for one embodiment may also comprise optional memory 834 to store suitable instructions to be executed by DSP 830 to adaptively control the linefeed on subscriber loop 120 in performing the battery feed function to account for variations in load on subscriber loop 120. Memory 834 may be used to store, for example, suitable instructions to adaptively control the linefeed on subscriber loop 120 in accordance with flow diagram 200 of FIG. 2 or with flow diagram 500 of FIG. 5.

[0112] Memory 834 for one embodiment may also be used to store suitable instructions to be executed by DSP 830 to perform one or more other functions, such as one or more other BORSCHT functions in addition to the battery feed function. Memory 834 for one embodiment may also be used to store one or more suitable values for one or more control parameters for one or more functions performed by DSP 830.

[0113] Memory 834 may comprise any suitable one or more non-volatile and/or volatile memories including, for example, flash memory, read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), a battery-backed random access memory (RAM), random access memory (RAM), static random access memory (SRAM), dynamic random access memory (DRAM), and/or synchronous dynamic random access memory (SDRAM).

[0114] Although described in the context of instructions stored in memory 834 to be executed by DSP 830, any suitable machine-executable instructions to adaptively control a linefeed on a subscriber loop to account for variations in load on the subscriber loop may be stored on any suitable machine-readable medium, such as a hard disk device, a floppy disk or diskette device, an optical disk device such as a compact disc (CD) or digital versatile disc (DVD) device for example, a Bernoulli disk device such as a Jaz or Zip disk device for example, a flash memory device, a file server device, and/or any other suitable memory device. For one embodiment, suitable instructions stored in this manner may be transmitted to SLIC 100 through DSP interface 832 for execution by DSP 830 and/or for storage in memory 834.

[0115] DSP 830 for one embodiment may also have a digital voiceband interface 836 to communicate digitized voiceband data to switching network 110 where switching network 110 is a digital switching network and have a digital voiceband interface 838 coupled to linefeed driver circuitry 710 and to subscriber loop sensing circuitry 720 for bi-directional voiceband data transfer between DSP 830 and subscriber loop 120.

[0116] DSP 830 for one embodiment is implemented on an integrated circuit. At least a portion of linefeed driver circuitry 710 and/or subscriber loop sensing circuitry 720 for one embodiment may also be implemented on the integrated circuit.

[0117] SLIC 100 for one embodiment may be implemented by programming or controlling any suitable SLIC architecture, including a suitable existing SLIC architecture, to adaptively control a linefeed on a subscriber loop to account for variations in load on the subscriber loop. As one example, SLIC 100 may be implemented by controlling an Si321x ProSLIC™ Programmable CMOS SLIC/Codec with Ringing and Battery Voltage Generation, manufactured by Silicon Laboratories, Inc. of Austin, Tex., to adaptively control a linefeed on a subscriber loop to account for variations in load on the subscriber loop.

[0118] In the foregoing description, one or more embodiments of the present invention have been described. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit or scope of the present invention as defined in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. A method comprising: generating a linefeed on a subscriber loop to generate a current on the subscriber loop; identifying a relationship between a voltage on the subscriber loop and a threshold voltage; modifying a current limit for the subscriber loop in response to the identifying; and controlling the linefeed on the subscriber loop to modify the current on the subscriber loop in response to the modifying.
 2. The method of claim 1, wherein the generating comprises controlling a current source to generate a generally constant current on the subscriber loop.
 3. The method of claim 1, wherein the identifying comprises identifying a relationship between a metallic voltage between a first line and a second line of the subscriber loop and the threshold voltage.
 4. The method of claim 1, wherein the identifying comprises identifying a relationship between the voltage on the subscriber loop and a range of voltages.
 5. The method of claim 1, wherein the modifying comprises increasing or decreasing the current limit by a predetermined amount.
 6. The method of claim 1, wherein the modifying comprises increasing the current limit if the voltage on the subscriber loop is less than the threshold voltage.
 7. The method of claim 1, wherein the modifying comprises decreasing the current limit if the voltage on the subscriber loop is greater than the threshold voltage.
 8. The method of claim 1, wherein the identifying comprises identifying a relationship between the voltage on the subscriber loop and a range of voltages; and wherein the method comprises modifying the range of voltages in response to the identifying.
 9. The method of claim 8, wherein the modifying the range of voltages comprises decreasing an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is less than the lower threshold voltage.
 10. The method of claim 8, wherein the modifying the range of voltages comprises increasing an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is greater than the upper threshold voltage.
 11. The method of claim 8, wherein the modifying the range of voltages comprises decreasing the difference between an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is less than the lower threshold voltage.
 12. An apparatus comprising: linefeed driver circuitry to generate a linefeed on a subscriber loop to generate a current on the subscriber loop; subscriber loop sensing circuitry to sense one or more voltages on the subscriber loop; and adaptive linefeed control circuitry to monitor a voltage based on the sensed voltage(s), to identify a relationship between the monitored voltage and a threshold voltage, to modify a current limit for the subscriber loop in response to the identifying, and to control the linefeed on the subscriber loop to modify the current on the subscriber loop in response to the modifying.
 13. The apparatus of claim 12, wherein the linefeed driver circuitry comprises a current source to generate a generally constant current on the subscriber loop.
 14. The apparatus of claim 12, wherein the adaptive linefeed control circuitry is to monitor a metallic voltage between a first line and a second line of the subscriber loop.
 15. The apparatus of claim 12, wherein the adaptive linefeed control circuitry is to identify a relationship between the monitored voltage and a range of voltages.
 16. The apparatus of claim 12, wherein the adaptive linefeed control circuitry is to modify the current limit for the subscriber loop by increasing or decreasing the current limit by a predetermined amount.
 17. The apparatus of claim 12, wherein the adaptive linefeed control circuitry is to modify the current limit for the subscriber loop by increasing the current limit if the monitored voltage is less than the threshold voltage.
 18. The apparatus of claim 12, wherein the adaptive linefeed control circuitry is to modify the current limit for the subscriber loop by decreasing the current limit if the monitored voltage is greater than the threshold voltage.
 19. The apparatus of claim 12, wherein the adaptive linefeed control circuitry is to identify a relationship between the monitored voltage and a range of voltages and is to modify the range of voltages in response to the identifying.
 20. The apparatus of claim 19, wherein the adaptive linefeed control circuitry is to decrease an upper threshold voltage and a lower threshold voltage for the range of voltages if the monitored voltage is less than the lower threshold voltage.
 21. The apparatus of claim 19, wherein the adaptive linefeed control circuitry is to increase an upper threshold voltage and a lower threshold voltage for the range of voltages if the monitored voltage is greater than the upper threshold voltage.
 22. The apparatus of claim 19, wherein the adaptive linefeed control circuitry is to decrease the difference between an upper threshold voltage and a lower threshold voltage for the range of voltages if the monitored voltage is less than the lower threshold voltage.
 23. The apparatus of claim 12, wherein the adaptive linefeed control circuitry is implemented with a digital signal processor.
 24. The apparatus of claim 12, comprising programmable registers to store values to identify the current limit and the threshold voltage.
 25. An apparatus comprising: means for generating a linefeed on a subscriber loop to generate a current on the subscriber loop; means for identifying a relationship between a voltage on the subscriber loop and a threshold voltage; means for modifying a current limit for the subscriber loop in response to the identifying; and means for controlling the linefeed on the subscriber loop to modify the current on the subscriber loop in response to the modifying.
 26. The apparatus of claim 25, wherein the identifying means comprises means for identifying a relationship between the voltage on the subscriber loop and a range of voltages.
 27. The apparatus of claim 25, wherein the modifying means comprises means for increasing or decreasing the current limit by a predetermined amount.
 28. The apparatus of claim 25, wherein the identifying means comprises means for identifying a relationship between the voltage on the subscriber loop and a range of voltages; and wherein the apparatus comprises means for modifying the range of voltages in response to the identifying.
 29. A machine-readable medium to store machine-executable instructions which, if executed by a machine, perform a method comprising: generating a linefeed on a subscriber loop to generate a current on the subscriber loop; identifying a relationship between a voltage on the subscriber loop and a threshold voltage; modifying a current limit for the subscriber loop in response to the identifying; and controlling the linefeed on the subscriber loop to modify the current on the subscriber loop in response to the modifying.
 30. The machine-readable medium of claim 29, wherein the identifying comprises identifying a relationship between a metallic voltage between a first line and a second line of the subscriber loop and the threshold voltage.
 31. The machine-readable medium of claim 29, wherein the identifying comprises identifying a relationship between the voltage on the subscriber loop and a range of voltages.
 32. The machine-readable medium of claim 29, wherein the modifying comprises increasing or decreasing the current limit by a predetermined amount.
 33. The machine-readable medium of claim 29, wherein the modifying comprises increasing the current limit if the voltage on the subscriber loop is less than the threshold voltage.
 34. The machine-readable medium of claim 29, wherein the modifying comprises decreasing the current limit if the voltage on the subscriber loop is greater than the threshold voltage.
 35. The machine-readable medium of claim 29, wherein the identifying comprises identifying a relationship between the voltage on the subscriber loop and a range of voltages; and wherein the method comprises modifying the range of voltages in response to the identifying.
 36. The machine-readable medium of claim 35, wherein the modifying the range of voltages comprises decreasing an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is less than the lower threshold voltage.
 37. The machine-readable medium of claim 35, wherein the modifying the range of voltages comprises increasing an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is greater than the upper threshold voltage.
 38. The machine-readable medium of claim 35, wherein the modifying the range of voltages comprises decreasing the difference between an upper threshold voltage and a lower threshold voltage for the range of voltages if the voltage on the subscriber loop is less than the lower threshold voltage. 